The present invention relates to a circuit board in which layers are electrically connected using a conductor such as a conductive paste and to a method of manufacturing the same.
Recently, the applicant has proposed a multilayer circuit board in which layers are electrically connected using a conductive paste (in Japanese Patent No. 2,601,128). FIG. 8 shows a method of manufacturing the multilayer circuit board. As shown in FIG. 8(a), release films 501 made of polyester or the like are laminated on both surfaces of a porous substrate 502 obtained by impregnating aromatic polyamide fibers with a thermosetting epoxy resin. As shown in FIG. 8(b), through holes 503 are formed at predetermined positions in the porous substrate 502 by a laser processing method. Then, the through holes 503 are filled with a conductive paste 504 as shown in FIG. 8(c). As a filling method, the porous substrate 502 with the through holes 503 is placed on a table of a screen printing machine and the conductive paste 504 is printed directly from the top of one of the release films 501. In this case, the release film 501 at the printed side serves as a print mask and to prevent the surface of the porous substrate 502 from being polluted. Then, the release films 501 are peeled off from both the surfaces of the porous substrate 502. As a next step, metal foils 505 such as copper foils are laminated on both the surfaces of the porous substrate 502. In this state, it is heated and pressurized, thus bonding the porous substrate 502 and the metal foils 505 as shown in FIG. 8(d). In this process, the porous substrate 502 is compressed to be made thin. Simultaneously, the conductive paste 504 within the through holes 503 also is compressed, and a binder constituent contained in the conductive paste 504 is forced out, thus strengthening the adhesion between conductive constituents and between the conductive constituents and the metal foils 505. As a result, the conductive material contained in the conductive paste 504 becomes dense, and thus layers are electrically connected to each other. After that, a thermosetting resin that is a constituent of the porous substrate 502 and the conductive paste 504 are cured. Finally, as shown in FIG. 8(e), the metal foils 505 are selectively etched in a predetermined pattern, thus completing a double-faced circuit board.
However, in the aforementioned configuration and manufacturing method, when the through holes 503 are formed to be minute, the initial connection resistance increases and greatly varies. Further, the connection resistance varies depending on reliability tests such as a temperature cycling test or a pressure cooker test, which has been a problem. The problem is caused by the aspect ratio, which is the ratio of the diameter of the through holes 503 and the thickness of the porous substrate 502, approaches 1 when the through holes 503 are formed to be minute, and therefore the compressibility required for stabilizing the electric connection cannot be obtained.
In the process of peeling off the release films 501, when the through holes are decreased in diameter, the influence of the release film cannot be ignored at the ends of the through holes. In peeling off the release films, the conductive paste 504 is removed by the release films, thus preventing the through holes from being filled with the conductive paste, which has been another problem.
Bearing the aforementioned problems in mind, the present invention aims to provide a circuit board that enables minute via holes to be obtained with high reliability using a conductive material such as a conductive paste and a method of manufacturing the same.
In order to solve the aforementioned problems, in a circuit board of the present invention, through holes formed in a thickness direction of an electrical insulating substrate are filled with a conductor, and wiring layers formed on both the surfaces of the electrical insulating substrate in a predetermined pattern are electrically connected by the conductor. The circuit board is characterized in that adhesive layers are formed on both the surfaces of the electrical insulating substrate and at least one wiring layer is embedded in one of the adhesive layers. In such a configuration, the conductor within the through holes is compressed sufficiently and thus minute via holes can be formed with high reliability. That is to say, since at least one wiring layer is embedded in one of the adhesive layers, the conductor within the through holes is compressed sufficiently. As a result, a conductive constituent of the conductor becomes dense, thus enabling via-hole connection with a low initial connection resistance and high reliability,
It is preferred to use a conductive paste as the conductor, since a resinous constituent contained in the conductive paste is forced out from the through holes when the conductive paste within the through holes is compressed and the conductive constituent contained in the conductive paste becomes dense, thus obtaining the via-hole connection with a low initial connection resistance and high reliability more easily.
When the through holes in the topmost layer are covered with a wiring layer, the conductor filling the through holes is not exposed at the surface. Consequently, it is effective to provide such through holes to the top layer of a substrate.
When the wiring layers are formed so that a part of the respective through holes is exposed and such wiring layers are used as inner layers, landless via holes in which via holes are compressed by a smaller wiring than the diameter of the via holes can be obtained. Thus, further minute wiring can be formed.
When at least the surface of each wiring layer facing the through holes is processed to be rough, the contact area between the wiring layer and the conductor increases, and the adhesion between the wiring layer and the adhesive layer also is increased. Therefore, the process is effective for further improving the reliability of minute via holes.
A first method of manufacturing a circuit board according to the present invention comprises: providing through holes in an electrical insulating substrate having adhesive layers formed on its both surfaces; filling the through holes with a conductive paste; superposing a supporting base, on which a wiring layer has been formed in a predetermined pattern, at least on one surface of the electrical insulating substrate; embedding the wiring layer in an adhesive layer by compressing the electrical insulating substrate, on which the supporting base has been superposed, by heating and pressurization; and removing the supporting base while leaving the wiring layer. The method enables a circuit board with via holes having high reliability in connection with a minute wiring layer to be provided by such an easy method that the patterned wiring layer is supported by the laminated supporting base that is removed after press.
A second method of manufacturing a circuit board according to the present invention comprises: laminating release films, each of which has an adhesive layer on its one surface, on both surfaces of an electrical insulating substrate so that each surface of the electrical insulating substrate comes into contact with the adhesive layer; providing through holes in the electrical insulating substrate having the release films; filling the through holes with a conductive paste; peeling off the release films while leaving the adhesive layers in the electrical insulating substrate; superposing a supporting base, on which a wiring layer has been formed in a predetermined pattern, at least on one surface of the electrical insulating substrate; embedding the wiring layer in the adhesive layer by compressing the electrical insulating substrate on which the supporting base has been superposed by heating and pressurization; and removing the supporting base while leaving the wiring layer. This method can avoid such a difficulty in manufacture as to form thin semi-cured adhesive layers on both surfaces of an electrical insulating substrate simultaneously. By such an easy method in which a supporting base on which a wiring layer has been formed is laminated and pressed and then is removed, a circuit board with via holes having high reliability in connection with a minute wiring layer can be provided.
In the aforementioned first or second manufacturing method, when the electrical insulating substrate before being heated and pressurized is a composite material of a semi-cured thermosetting resin and a glass woven fabric and the adhesive layers are formed of the thermosetting resin, a conventional glass epoxy composite material can be used. Therefore, an extra step for providing the adhesive layer is not necessary, resulting in easy manufacture of a circuit board.
The electrical insulating substrate before being heated and pressurized may be formed of a film comprising an organic material as a main constituent and the adhesive layers may be formed of a semi-cured organic resin. By selecting a material having high heat resistance and high stiffness as a film material, characteristics suitable for mounting semiconductors can be provided. The material of the adhesive layer can be freely selected considering electric insulation and a property capable of receiving an object to be embedded. Thus, a circuit board with high performance can be obtained. Furthermore, a thin film with a uniform composition can be produced, which is convenient for forming via holes with a minute diameter.
When the thickness of the adhesive layer provided on the surface of the electrical insulating substrate before being heated and pressurized is substantially the same as or thinner than that of the wiring layer to be embedded in the adhesive layer, the wiring layer can-be embedded so as almost to reach the electrical insulating substrate, thus minimizing the decrease in compressive force to the conductive paste due to spread of the adhesive layer in the horizontal direction when being compressed.
When the electrical insulating substrate before being heated and pressurized has spaces capable of receiving constituent materials of the adhesive layers, the electrical insulating substrate receives the constituent materials of the adhesive layers that have been melted by being heated and pressurized. Consequently, distortion of the electrical insulating substrate that is caused by embedding the wiring layer can be restrained.
When the electrical insulating substrate before being heated and pressurized has minute pores through which the constituent materials of the adhesive layers provided on both the surfaces of the electrical insulating substrate can pass, the constituent materials of the adhesive layers that have been melted by being heated and pressurized can flow up and down in the electrical insulating substrate. As a result, the distortion of the electrical insulating substrate can be restrained further.
It is preferable that in the process of removing the supporting base while leaving the wiring layer, the supporting base is selectively melted and removed. By melting and removing the supporting base, a mechanical external force is not applied to the wiring layer. Therefore, a circuit board having a minute wiring layer that is not disconnected or deformed can be manufactured with a good yield. In addition, even a circuit board having a large area can be manufactured easily.
In the aforementioned first or second method of manufacturing a circuit board, it is preferable that the wiring layer and the supporting base are formed of respective materials that enable them to be removed selectively. According to this configuration, after embedding the wiring layer in the adhesive layer, the supporting base can be removed easily while leaving only the wiring layer.
In this case, it is preferable that the wiring layer and the supporting base are formed so as to be removed selectively by different etchants. This configuration prevents the wiring layer from being removed simultaneously by over-etching when removing the supporting base by etching after embedding the wiring layer in the adhesive layer. Therefore, the wiring layer having a minute pattern can be left in the adhesive layer with a good yield. Further, a sufficient difference in etching speed, for example, a difference of about several times is enough for removing one of them selectively by etching.
Further, in the first or second method of manufacturing a circuit board, it is preferable that at least one etching-stopper layer is formed on the surface of the supporting base, a wiring layer is provided on the surface of the etching-stopper layer, and the wiring layer and at least the etching-stopper layer are formed of respective materials that enable them to be removed selectively. According to this configuration, selectivity in removing only one selected from the supporting base and the wiring layer by etching can be improved. As a result, after embedding the wiring layer in the adhesive layer, the supporting base can be removed easily while leaving only the wiring layer. More particularly, in removing the supporting base by etching, the wiring layer can be prevented from being removed simultaneously due to over-etching. Therefore, the wiring layer having a minute pattern can be left in the adhesive layer with a good yield.
In this case, it is preferable that at least the wiring layer and the etching-stopper layer are formed so as to be removed selectively by different etchants. According to this configuration, even if the supporting base and the wiring layer are formed of respective materials that are removed by the same etchant, etching with an etchant capable of removing only the etching-stopper layer selectively enables the supporting base to be removed together with the etching-stopper layer, thus leaving only the wiring layer.
In the first or second method of manufacturing a circuit board, it is preferable that the supporting base on which the wiring layer has been formed is obtained by: forming an insulating-material pattern on the surface of the supporting base or the surface of the etching-stopper layer formed on the supporting base; and forming a wiring layer with a desired pattern by allowing a conductive material to adhere by plating to the area where the surface of the supporting base or the surface of the etching-stopper layer is exposed. According to such a configuration, a wiring layer with a desired pattern can be obtained easily.
In this case, the insulating-material pattern can be formed by allowing a photosensitive resin to adhere to the surface of the supporting base or the surface of the etching-stopper layer, then exposing it selectively according to the desired mask pattern, and developing it. Thus, a desired insulating-material pattern can be formed easily in a desired area.
When the xe2x80x9cplatingxe2x80x9d is electroplating, a conductive material easily can adhere selectively only to the exposed area where no insulating-material pattern has been formed. Consequently, a desired minute wiring pattern can be formed easily.
In the first or second method of manufacturing a circuit board, it is preferable that the supporting base on which the wiring layer has been formed is obtained by: laminating a substrate on the back surface of the supporting base using an adhesive that loses its adhesion at a predetermined temperature or higher; and forming a wiring layer on the surface of the supporting base directly or with an etching-stopper layer being sandwiched therebetween. Even if the supporting base is a thin film member, it can be reinforced by laminating the substrate having a sufficient strength for supporting the supporting base. When the wiring layer is formed after that, the process of forming the wiring layer can be carried out easily. In addition, by laminating the substrate using the adhesive that loses its adhesion at a predetermined temperature or higher, the supporting base and the substrate can be separated easily by heating after predetermined processes. As a result, the working efficiency is improved.
A multilayer circuit board according to a first configuration of the present invention is formed by laminating at least two electrical insulating substrates with an adhesive layer being sandwiched therebetween. The electrical insulating substrates have through holes formed in the thickness direction and filled with a conductor. The multilayer circuit board is characterized in that a wiring layer formed in a predetermined pattern is provided in the adhesive layer and the wiring layer is electrically connected to the conductors in both the electrical insulating substrates sandwiching the adhesive layer due to a compressive force applied in the laminated direction. According to such a configuration, a multilayer circuit board with minute via holes having high reliability can be provided.
A first method of manufacturing a multilayer circuit board according to the present invention is characterized by repeating the steps of: superposing a supporting base, on which a wiring layer has been formed in a predetermined pattern, on one surface of an electrical insulating substrate that has through holes filled with a conductive paste and adhesive layers on its both surfaces; embedding the wiring layer in an adhesive layer by applying a compressive force by heating and pressurization; and removing the supporting base while leaving the wiring layer. According to such a configuration, a simple method of manufacturing a multilayer circuit board can be provided.
In the aforementioned first method of manufacturing a multilayer circuit board, it is preferable that the wiring layer and the supporting base are formed of respective materials that enable them to be removed selectively. It also is preferable that at least one etching-stopper layer is formed on the surface of the supporting base, a wiring layer is provided on the surface of the etching-stopper layer, and the wiring layer and at least the etching-stopper layer are formed of respective materials that enable them to be removed selectively. Furthermore, it is preferable that the supporting base on which the wiring layer has been formed is obtained by: forming an insulating-material pattern on the surface of the supporting base or the surface of the etching-stopper layer formed on the surface of the supporting base; and forming a wiring layer with a desired pattern by allowing a conductive material to adhere by plating to the area where the surface of the supporting base or the surface of the etching-stopper layer is exposed. In addition, it is preferable that the supporting base on which the wiring layer has been formed is obtained by: laminating a substrate on the back surface of the supporting base using an adhesive that loses its adhesion at a predetermined temperature or higher; and forming a wiring layer on the surface of the supporting base directly or with an etching-stopper layer being sandwiched therebetween. The aforementioned methods are preferable due to the same reasons as those in the case of the first or second method of manufacturing a circuit board.
A multilayer circuit board according to a second configuration of the present invention is characterized in that the wiring layer formed of the top layer of the multilayer circuit board according to the first configuration of the present invention and a wiring layer of the top layer of a core substrate having a predetermined number of insulating layers and wiring layers are electrically connected with an electrical insulating substrate, which has adhesive layers on its both surfaces and through holes filled with a conductor, being sandwiched therebetween, and at least one selected from the wiring layer of the top layer of the multilayer circuit board and the wiring layer of the top layer of the core substrate is embedded in an adhesive layer. According to such a configuration, a multilayer circuit board in which the wiring layer of the top layer of the core substrate and the wiring layer of the top layer of the first multilayer circuit board formed of a plurality of layers having minute via holes and minute wiring are electrically connected can be provided.
A second method of manufacturing a multilayer circuit board according to the present invention is characterized by comprising: superposing the multilayer circuit board according to the first configuration of the present invention on a core substrate having a predetermined number of insulating layers and wiring layers with an electrical insulating substrate, which has adhesive layers on its both surfaces and through holes filled with a conductive paste, being sandwiched therebetween; and embedding at least one selected from the wiring layer formed on the top layer of the multilayer circuit board and the wiring layer of the top layer of the core substrate in an adhesive layer by heating and pressurizing the core substrate and the multilayer circuit board that have been superposed with the electrical insulating substrate being sandwiched therebetween. According to such a configuration, a simple method of manufacturing a multilayer circuit board can be provided.
In the first or second method of manufacturing a multilayer circuit board, the electrical insulating substrate before being heated and pressurized may be a composite material of a semi-cured thermosetting resin and a glass woven fabric, and the adhesive layer may be formed of the thermosetting resin. Alternatively, the electrical insulating substrate before being heated and pressurized may be formed of a film comprising an organic material as a main constituent, and the adhesive layer may be formed of a semi-cured organic resin.
In the first or second method of manufacturing a multilayer circuit board, it is preferable that the thickness of each adhesive layer provided on the surfaces of the electrical insulating substrate before being heated and pressurized is substantially the same as or thinner than that of the wiring layer to be embedded in the adhesive layer.
In the first or second method of manufacturing a multilayer circuit board, it is preferable that the electrical insulating substrate before being heated and pressurized has spaces capable of receiving constituent materials of the adhesive layers. It is further preferable that the electrical insulating substrate before being heated and pressurized has minute pores through which the constituent materials of the adhesive layers can pass.
Furthermore, a multilayer circuit board according to a third configuration of the present invention is characterized in that the multilayer circuit board according to the first configuration of the present invention and a core substrate having a predetermined number of insulating layers and wiring layers are laminated with a substrate bonding body having through holes filled with a conductor being sandwiched therebetween, the wiring layer formed on the top layer of the multilayer circuit board and the wiring layer of the top layer of the core substrate are electrically connected via the conductor, and the substrate bonding body before being laminated has compressibility. According to such a configuration, a multilayer circuit board in which the wiring layer of the core substrate and the wiring layer of the first multilayer circuit board formed of a plurality of layers having minute wiring and minute via holes are electrically connected can be provided.
In the above, xe2x80x9cthe substrate bonding body has compressibilityxe2x80x9d means that, for example, the substrate bonding body is formed of a porous substrate having holes inside and thus has a property of being able to be compressed. In the case of using the porous substrate, a preferable porosity is 2-35% by volume. When the porosity is lower than that, the substrate bonding body is difficult to be compressed, thus causing increase in the electric connection resistance between the conductor and the wiring layer or a bad connection. On the other hand, when the porosity is higher than that, the substrate bonding body is further deformed in the vertical direction to the compression direction when being compressed and the conductive resin enters into the holes. Consequently, the conductive resin cannot be compressed sufficiently, thus increasing the electric connection resistance between the conductor and the wiring layer.
In the aforementioned third multilayer circuit board, it is preferable that the material forming the substrate bonding body is at least one material selected from resin-impregnated fiber sheet materials composed of a composite material of a glass-fiber nonwoven fabric or an organic-fiber nonwoven fabric, and a thermosetting resin. According to such a preferable configuration, electric and mechanical characteristics of the multilayer circuit board are further improved.
A third method of manufacturing a multilayer circuit board according to the present invention is characterized by comprising: superposing the multilayer circuit board according to the first configuration of the present invention on a core substrate with a predetermined number of insulating layers and wiring layers with a substrate bonding body that has through holes filled with a conductive paste and has compressibility being sandwiched therebetween; and electrically connecting the wiring layer., formed on the top layer of the multilayer circuit board and the wiring layer of the core substrate via the conductive paste by heating and pressurizing the multilayer circuit board and the core substrate that have been superposed with the substrate bonding body being sandwiched therebetween. Such a configuration enables a simple method of manufacturing a multilayer circuit board to be provided.
In the aforementioned third method of manufacturing a multilayer circuit board, it is preferable that the conductive paste filling the through holes in the substrate bonding body before being heated and pressurized protrudes from the surfaces of the substrate bonding body. According to such a preferable configuration, the both wiring layers can be electrically connected to each other stably via the conductive paste with a low resistance.
A second circuit board of the present invention includes an electrical insulating substrate, adhesive layers, wirings, and a conductor. The adhesive layers are provided on both surfaces of the electrical insulating substrate. The wirings are formed on both sides of the electrical insulating substrate. The conductor electrically connects the wirings on both sides of the electrical insulating substrate with each other. At least part of the wirings is embedded in the adhesive layers, and a volume of one of the adhesive layers provided on one of both surfaces of the electrical insulating substrate on a side of a smaller volume except a wiring volume is smaller than that of the other adhesive layer provided on the other surface.
The above-mentioned volume except a wiring volume can be defined as the product of the area of portions of the adhesive layer where the wiring is not embedded and the average thickness of portions, of the wiring, embedded in the adhesive layer. The circuit board of the present invention is configured so that the volume of the adhesive layer provided on the side of a smaller volume except a wiring volume is smaller than that of the adhesive layer provided on the other side. This can reduce the difference in stresses applied to both sides of the electrical insulating substrate by the adhesive layers in the case where patterns and densities of the wirings on both sides of the electrical insulating substrate are different from each other, when compared to the difference in a configuration in which adhesive layers with the same volume are provided on both sides of an electrical insulating substrate. Thus, the difference in strength of the connections between the conductor and the wirings on both sides of the electrical insulating substrate is reduced and therefore, the deformation of the electrical insulating substrate can be suppressed, thus reducing inner stress of the circuit board. As a result, a circuit board, allowing a stable interlayer connection to be obtained can be provided.
In the second circuit board with the configuration described above, preferably the distances between the electrical insulating substrate and surfaces of the wirings in contact with the conductor, are substantially the same on both sides of the electrical insulating substrate. According to this configuration, the strengths of the connections between the wirings and the conductor are substantially the same on both sides of the electrical insulating substrate. Thus, a circuit board allowing a stable interlayer connection to be obtained can be provided.
In the second circuit board with the configuration described above, preferably surfaces of the wirings in contact with the conductor are embedded in the adhesive layers. This configuration allows initial connection resistance to be low, the variation in the initial connection resistance to be reduced, and high connection reliability to be obtained.
In the second circuit board with the configuration described above, preferably the conductor is formed of a conductive paste. According to this configuration, filling of the conductive paste can be carried out by printing, and therefore the conductor can be formed easily.
In the second circuit board with the configuration described above, preferably the electrical insulating substrate is formed of a compound of fiber and thermosetting resin. According to this configuration, it is possible to provide a circuit board having an electrical insulating substrate with high stiffness and having a property allowing electronic circuit components to be mounted easily.
In the second circuit board with the configuration described above, preferably the electrical insulating substrate is formed of an organic resin film. According to this configuration, it is possible to provide a thin and flexible circuit board.
In the second circuit board with the configuration described above, preferably the adhesive layers are formed of thermosetting organic resin. According to this configuration, even when being subjected to a high temperature state, the adhesive layers are not softened. Therefore, a circuit board with connection stability that does not vary depending on heat history can be provided.
A multilayer circuit board of the present invention is characterized by including at least one second circuit board according to any one of the configurations described above. According to this configuration, it is possible to provide a multilayer circuit board allowing a stable interlayer connection to be obtained.
A method of manufacturing a circuit board according to the present invention includes: forming adhesive layers on both surfaces of an electrical insulating substrate; forming a conductor going through the electrical insulating substrate and the adhesive layers; attaching supporting bases with wirings formed thereon to the electrical insulating substrate by pressure so that at least part of the wirings is embedded in the adhesive layers; and removing the supporting bases while leaving the wirings. In the process of forming adhesive layers, a volume of one of the adhesive layers formed on one of both surfaces of the electrical insulating substrate on a side of a smaller volume except a wiring volume is smaller than that of the other adhesive layer formed on the other surface.
The above-mentioned volume except a wiring volume can be defined as the product of the area of portions of the adhesive layer where the wiring is not embedded and the average thickness of portions, of the wiring, embedded in the adhesive layer. In the case of a small volume except a wiring volume, when the supporting bases with the wirings formed thereon are attached to the electrical insulating substrate by pressure, an excess adhesive that cannot be received between the wirings hinders the attachment by pressure between the wirings and the conductor. According to the manufacturing method described above, however, the volume of one of the adhesive layers formed on the side of a smaller volume except a wiring volume is set to be smaller than that of the other adhesive layer formed on the other surface. Consequently, the difference in strength of the connections between the conductor and the wirings on both sides of the electrical insulating substrate is reduced and therefore, the deformation of the electrical insulating substrate after the process of attaching the supporting bases by pressure is suppressed, thus reducing the inner stress of the circuit board. As a result, it is possible to provide a circuit board allowing a stable interlayer connection to be obtained.
In the above-mentioned method of manufacturing a circuit board, preferably the process of forming a conductor includes forming a through hole going through the electrical insulating substrate and the adhesive layers and filling the through hole with a conductive paste.
According to this manufacturing method, filling of the conductive paste can be carried out easily by printing that is suitable for mass production. Consequently, a circuit board allowing a stable interlayer connection to be obtained can be provided at a low cost.